Any chance that unlocking the speed limit display in the HUD would be possible?

I think the F10 guys are still working on that. Sometimes it shows but is not sticky or sometimes they only get a solid icon with no data. However, you at least need ZDA - Drivers Assist (HW - cameras and processer) with Lane Departure and Active Blind Spot Monitoring.

So, for MY13 it is a moot point as ZDB adds the fwd camera with both Lane Departure and Speed Limit Info.

But, this seems to indicate the blind spot monitoring is done with PDC sensors and the surround cameras. So, what might be interesting is whether Active Blind Spot Monitoring info can be coded to show in HUD on MY12 if you have the Parking Pkg (ZPK). (You will not have the steering wheel vibration, as that is a separate HW piece as well.)